Remove -Wall from default build options
All bazel crosstools automatically pass -Wall to compiles. The order of
operations is:
- bazel crosstool flags
- `--host_copt` flags and other variations like `--host_cxxopt`
- the `copts` defined on the rule
Because of this when protobuf produces warnings, there's no way to
disable them from the consumer side if they are re-enabled later by
another `-Wall` flag.
Keith Smiley